Many communication devices include a balanced-to-unbalanced (balun) transformer to transform differential signals into single-ended signals, and/or to transform single-ended signals into differential signals. A balun typically includes two mutually coupled inductors (e.g., coils or windings) configured such that an input signal applied to the primary inductor induces an output voltage across the secondary inductor. In addition to transforming signals, baluns may also be used to provide impedance matching and filtering functions.
The quality factor (Q) of the balun may indicate the efficiency of energy transfer from the primary inductor to the secondary inductor. For example, a higher value of Q may indicate a higher voltage gain, higher energy transfer, and lower signal-to-noise ratio (SNR) for the balun, whereas a lower value of Q may indicate a lower voltage gain, lower energy transfer, and higher signal-to-noise ratio (SNR) for the balun.
As demands for higher bandwidth signal processing increases, so does the need for baluns to exhibit wider frequency bandwidths. The bandwidth of a balun may be increased by “de-Q'ing” the balun. For example, the Q of the balun may be decreased by providing a de-Q'ing resistor across the primary inductor, thereby widening the balun's frequency response (e.g., bandwidth) by increasing the balun's input impedance. Although effective in widening the frequency response of the balun, de-Q'ing the balun introduces unwanted losses, reduces gain, and may increase power consumption.
Thus, there is a need to widen the bandwidth of a balun without introducing additional losses, without reducing gain, and without increasing power consumption. In addition, it is desirable to achieve these goals without increasing circuit area of the balun and without increasing the complexity of associated control circuits and/or calibration circuits.